"eval" in ghc(i)?

[Geoffrey Alan Washburn]

Lemmih wrote:
> On 5/4/06, Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
>> geoffw:
>> >
>> >       I have an application written in OCaml that I'm interested in
>> >       porting over to Haskell, and I was wondering what the best way 
>> to replace
>> > the following OCaml function would be:
>> >
>> > Toploop.initialize_toplevel_env();;
>> >
>> > let eval txt = let lb = (Lexing.from_string txt) in
>> >  let phr = !Toploop.parse_toplevel_phrase lb in
>> >  Toploop.execute_phrase true Format.std_formatter phr;;
>> >
>> > eval "let add1 x = x +1;;";;
>> > eval "add1 2;;";;
>> >
>> > Where I would like to be able to "eval" Haskell-code instead.  It looks
>> > like I might be able to achieve something like this using hs-plugins,
>> > but it looks a bit more complex.  Is hs-plugins the best choice for 
>> this
>> > kind of "meta"-programming?  I'm pretty sure Template Haskell will not
>> > work for me, at least as I understand it I can only manipulate program
>> > fragments that will be compiled later and as such that it will not be
>> > possible to execute them until the next "phase".
>>
>> You can do some forms of runtime metaprogrammign with hs-plugins, yes.
>> E.g.
>>
>> Prelude System.Eval.Haskell> v <- eval "1 + 2 :: Int" [] :: IO (Maybe 
>> Int)
>> Prelude System.Eval.Haskell> v
>> Just 3
>>
>> Prelude System.Eval.Haskell> mf <- eval "\\x -> x + 1 :: Int" [] :: IO 
>> (Maybe (Int -> Int))
>> Prelude System.Eval.Haskell> let f = fromJust mf
>> Prelude System.Eval.Haskell> :t f
>> f :: Int -> Int
>> Prelude System.Eval.Haskell> f 7
>> 8
>>
>> So if your program critically relies on this its possible to do.
> 
> You can also use the GHC library:
> Prelude> :m GHC
> Prelude GHC> GHC.init (Just 
> "/home/david/coding/haskell/ghc/usr/lib/ghc-6.5")
> Prelude GHC> session <- newSession Interactive
> Prelude GHC> setSessionDynFlags session =<< initPackages =<<
> getSessionDynFlags session
> Prelude GHC> setContext session [] [mkModule "Prelude"]
> Prelude GHC> runStmt session "let add1 x = x + 1"
> Prelude GHC> runStmt session "add1 2"
> 3
> Prelude GHC> :q
> Leaving GHCi.

	Thanks!  I think this is a bit closer to what I'm looking for than the 
hs-plugins eval.  It is possible to get runStmt to output the result of 
the session to a string rather than stdout?